We have isolated a new tryptophan derivative from human urine. It was named as substance X (S.X). The compoundconsists of tryptophan and sugar residue and has molecular weight of 366.In this study, we proposed that a measurement of S.X can be a new reliable marker of renal function.Unlike creatinine (Cr.), serum levels and renal outputs of S.X are stable in healthy subjects from age 1 to age 60. S.X is elevated in serum of subjects older than 60 year old. This may indicate the decrease of renal function due to ageing. Relationships between S.X and Cr.and S.X clearance and Cr.clearance were excellent, Y=112.5+88.1x, r=0.842, y=0.23+0.59x, r=0.920, respectively. Unlike Cr.level, S.X level is not affected by the muscle volume. In this study, we used two parameters, Cr.output and body mass index, instead direct measurement of the muscle volume. In animal model, S.X clearance showed more close relationship to inulin clearance than Cr.clearance (Csx=0.028+0.945xCin, r=0.976 vs Ccr=-0.079+1.34xCin, r=0.959). Measurement of S.X is more reliable than that of Cr.in estimation of glomerular filtration rate.Besides serum and urine, S.X was observed in anmiotic fluid and cerebral spinal fluid. S.X in amniotic fluid was increased with gestation period. S.X level in cerebral spinal fluid is higher than that in serum. This may suggest that S.X has biological role in central nervous system. The existence of S.X was confirmed in other species, like lamprey, chicken egg, mouse, rat, and dog. This means that S.X plays also some physiological role.